The present invention relates generally to well treating solutions for changing the relative permeability of a formation being treated to water and, more specifically, to a well treating solution for reducing water production problems on producing oil and gas wells.
Many oil producing zones have variable permeability both vertically and horizontally. In zones where permeability stratification exists, the displacing water used in producing oil sweeps fastest through the more permeable zones so that much of the oil in the less permeable zones must be produced at high water-oil ratios. Production of unwanted water has consistently burdened the industry since the cost of lifting, separating, and disposing of produced water makes it less profitable to produce oil. In the past, it has been stated that many oil wells produce a gross effluent comprising 80 to 98 percent by volume water and only 2 to 20 percent by volume of oil. Because of this, most of the pumping energy expended in producing a well is spent in lifting water from the well. The additional expense of separating and disposing of separated foul water continues to rise, especially in view of increasing environmental concerns. Similar problems exist with producing gas wells.
Prior attempts to reduce water production in producing oil and gas wells have included the injection into the formation of an aqueous solution thickened with a gelling polymer followed by a cross-linking ionic solution used to further cross-link the polymer. Inert materials such as ground walnut hulls, asbestos, or the like were sometimes added to the polymer solutions to act as a filler for a filter cake to lower the rate of water production. Other prior techniques included forming solid "plugs" within the formation to control water flow.
One problem with the prior techniques has been that polymeric solutions that have cross-linked sufficiently to form a gel are difficult to circulate or to inject into an undergound formation with sufficient penetration to achieve adequate results. The cross-linked polymeric systems were complicated to use and required close timing to achieve adequate gellation without exceeding the available pumping power of the injection unit. Certain of the prior materials have been extremely shear sensitive. Other prior materials have been ineffective because of toxicity problems of the material utilized.